The images to be displayed on a display of a television or a personal computer are generated based on the data that represents the color of each pixel by numerical values.
There are various methods, such as the RGB method, YUV method, and HSV method, which represent the color of each pixel by numerical values.
For example, the RGB method represents, by a numerical value, the intensity of each of the three primary colors (red, green, and blue) for each pixel. The greater the numerical value is, the closer to white the color represented by the numerical value is; and the smaller the numerical value is, the closer to black the color is. Typically, the RGB format is used in displays of personal computers. Therefore, text image data and graphics image data generated by using a personal computer are described in the RGB format.
In the YUV format, the color of one pixel if represented by a variable Y, a color difference U, and a color difference V, where the variable Y indicates the luminance (brightness), the color difference U indicates a difference between the luminance and the blue component, and a color difference V indicates a difference between the luminance and the red component. The YUV format has several types such as YUV 420 format and YUV 422 format, where the YUV 420 format reduces the data amount by a structure for thinning out the data in which four adjacent pixels share a set of color difference data. These YUV formats make use of a characteristic of the human eyes that they react to the luminance more easily than to the color, and they are suited for images such as natural images (for example, a photo of a landscape or a photo of a person) in which the color gradually changes. The YUV formats are often used in JPEG or MPEG, and are used in the digital TV broadcasting.
It is considered that the editing, such as enlarging, reducing, and rotating, and/or image quality adjustment is performed onto an image that has been described in one of the various data formats, the optimum editing algorithm differs for each data format.
For example, the text image data or graphics image data described in the RGB format may be enlarged by copying the data of adjacent pixel (zero-hold method), or by adjusting the edges in the shape of staircase (linear interpolation method).
Also, when the photo data or the like described in the YUV format is enlarged, a more complicated process such as the adjustment of the gradation change in color needs to be performed, as well as merely copying the data of adjacent pixel or adjusting the edges. Non-Patent Document 1 identified below discloses the editing of image data in detail.
Meanwhile, there may be a case where a plurality of pieces of image data respectively described in different data formats are combined into one piece of image data representing an image for one screen. In one example of such cases, graphics image data in the RGB format and photo data in the YUV format are displayed in a home page.
When a plurality of pieces of image data respectively described in different data formats are combined into one piece of image data, all pieces of pixel data must be described in a single format. After all pieces of pixel data are unified into one data format, it is impossible to know what data format each pixel was in before combining.    Patent Document 1: Japanese Patent Application Publication No. S61-156091    Non-Patent Document 1: Hitoshi KIYA, “Yokuwakarudougaseishiga no shori gijutsu (For Full Understanding of the Technology for Processing Moving/Still Images)”, Sep. 1, 2004, CQ Publishing Co., Ltd.    Non-Patent Document 2: Hideyuki TAMURA, “Konpyuta gazou shori (Computer Image Processing)”, Dec. 20, 2002, Ohmsha, Ltd.